Slope failure and subsequent debris flow occurrence on May 2017 at Iiyama, Nagano, Japan

Shinya HIRAMATSU, Koji ISHIDA, Akito KANAZAWA, Yutaka GONDA, Yoji SAWA, Daizo TSUTSUMI, Takahiko NAGAYAMA,
Taijiro FUKUYAMA, Masaaki MANTOKU, Tsutomu MIIKE, Atsushi MORISHITA, Koichi YAJIMA and Yasuhiro YAMADA

On 19 May 2017, slope failure occurred and a landslide dam formed at Idegawa watershed, Iiyama city, Nagano prefecture. After three days interval, debris flow occurred several times at Degawa River then partially reached Chikumagawa River. Although there were no loss of lives or property damage, an evacuation advisory has been issued on 20 May, then changed evacuation order on 22 May. After 23 June, this area is under evacuation advisory again. After the disaster, research team was immediately organized by The Shin]etsu Branch of Japan Society of Erosion Control Engineering. In this report, we described the magnitude and the generation processes of landslide and subsequent debris flow occurrence based on a field survey, weather observation data and the airborne laser scanning and videos. Furthermore, we evaluated the effect of the sabo]related facilities and overviewed the monitoring system and emergency measures.

Key wordsFlarge]scale slope failure, landslide dam, debris flow, Idegawa watershed, Iiyama city